1. Field of the Invention
Zinc sulfide is a durable material which intrinsically is transparent to relatively long electromagnetic wavelengths in the far-infrared range. These properties contribute to its use in applications which require infrared transmission capability such as in infrared detectors and missile domes. Zinc sulfide forms or articles are typically produced by chemical vapor deposition (CVD) or hot pressing techniques. These techniques result in forms which are generally opaque and not functionally transparent in the visible or near-infrared regions of the electromagnetic spectrum. Hot-isostatic pressing (HIP) has been found to sufficiently improve the transparency of zinc sulfide articles in the visible and near-ultraviolet regions that these forms can be used in applications requiring multi-spectral capability, such as in tank and aircraft windows. However, the visual transparency and clarity of such articles is less than desirable restricting the use of these articles in applications which require optical clarity. This invention relates to improvements in the CVD process which result in articles having significantly improved visual clarity after HIP treatment.
2. Description of Related Art
Chemical vapor deposition has been used to produce zinc sulfide in bulk form or in the form of a layered coating on a substrate. Typically such production involves the reaction of H.sub.2 S with vaporized zinc in proximity to either a substrate or a mandrel box on which the zinc sulfide deposits, such as is described in U.S. Pat. No. 5,686,195. The zinc sulfide deposits produced by this CVD method generally exhibit poor transmission in the visible and near-infrared regions of the electromagnetic spectrum. The transmission properties in these regions can be substantially improved by hot isostatic pressing (HIP) of the CVD produced form as described in U.S. Pat. No. 4,944,900.
A moderately clear ZnS has previously been commercially produced by a two-step process. First, elemental zinc vapors are reacted with hydrogen sulfide at a H.sub.2 S/Zn molar ratio of 1, a mandrel temperature of 735.degree. C. and an absolute pressure of 35 torr, in a CVD reactor. Zinc sulfide is deposited on the mandrel until a deposit of the desired thickness is produced. The deposit is separated from the mandrel to provide a zinc sulfide form which is then HIP treated for up to 100 hours at 900-1000.degree. C. and pressures of 15,000-30,000 psi. While the clarity of an image transmitted through the zinc sulfide form is substantially improved by the HIP treatment, it is not adequate for applications requiring image transmission through relatively thick forms or for optical applications with prisms or the like. Such clarity of image is related to the forward scatter value of a beam from a He--Ne laser as measured by a scatterometer. The forward scatter of zinc sulfide products of the above noted HIP treatment have typically varied between 10 and 40% cm.sup.-1. For imaging applications, such as required in military aircraft and vehicles, the scatter value should be as low as possible, many such applications requiring scatter values less than 7% cm.sup.-1. Previous attempts to improve these scatter values resulted in increased inclusions in the material, which degrade its transmission and optical quality, and also produced bowing, or induced curvature, in the material, which can cause cracking and reduce the yield of flat plates.
Accordingly, a need has existed for a reliable method of producing zinc sulfide articles having improved transmission and clarity in the visible and near-infrared electromagnetic wavebands, while avoiding increasing inclusions and/or bowing in the product articles. The present invention provides zinc sulfide articles having enhanced extended transparency in the visible range which permits their use in optical applications, such as for lenses, prisms, windows, etc., and other applications which take advantage of its high refractive index.
Moreover, bulk materials derived from CVD produced deposits often encounter induced curvature, or bowing, which, as with the zinc sulfide products discussed above, can result in cracks appearing in the product as well as reducing the yield of flat plates or articles. Accordingly, a general need exists for improved CVD processes which reduce the incidence and severity of bowing in the bulk product.